1. Field of the Invention
An aspect of the present invention relates to a laser diode array, an optical scanning device, and a printing apparatus, and particularly relates to the laser diode array including an edge emitting type laser array chip which includes three or more laser diodes and emits plural laser beams, and the optical scanning device and the printing apparatus mounted with the laser diode array.
2. Description of the Related Art
An exemplary configuration of an edge emitting type laser diode array is shown in FIGS. 1 to 3 (see also JP-H06-038542-B).
In FIG. 3, a laser diode array including an edge emitting type laser array chip 1 in which plural laser diodes electrically separated each other by separation grooves 7 are formed in an array shape on a main surface 6a of a semiconductor substrate 6 made of, for example, gallium and arsenic. As shown in FIG. 2, the laser array chip 1 are downwardly faced so that electrodes 8 thereof faces to a mounting area of the sub-mount 2 for mounting the laser array chip 1, and the laser diodes are mounted on the mounting area through the electrodes 8. The sub-mount 2 is made of, for example, silicon carbide. In this case, a common electrode 9 is provided on the bottom surface of the semiconductor substrate 6. In the mounting area of sub-mount 2 for the laser array chip 1, plural electrodes (not shown) in which solder layers are formed on surfaces thereof and arranged at a position opposed to the electrodes 8 of the laser array chip 1. Accordingly, the plural electrodes (not shown) of the sub-mount 2 are electrically and mechanically connected to the electrodes 8 of the plural laser diodes through the solder layers.
As shown in FIG. 1, in an area different from the mounting area of the sub-mount 2, bonding pads for connecting plural bonding wires are formed at positions at which a laser beam is not blocked at the time of connecting wires. The bonding pads connect the other ends of the bonding wires to one ends of plural leads 4 which are formed through a flange 5 of a package and are electrically isolated from the flange 5, thereby forming conductive connection from the outside of the package to the semiconductor laser diodes.
The sub-mount 2 is connected to a heat sink 3 made of a material such as copper having high thermal conductivity in order to suppress heat generation of the laser diodes. In addition, the heat sink 3 is mechanically connected to the flange 5 through solder.
A laser beam is emitted upward from the end surface of the laser array chip 1 in FIGS. 1 and 2.
JP-2003-347657-A discloses a laser diode array in which semiconductor laser diodes are fixed to a heat sink, and in which one of a first pair of electrodes is formed on a contact layer, the other of the first pair of electrodes is formed on the same side as the one electrode on a substrate, the heat sink including a second pair of electrodes is provided on a heat sink substrate, the semiconductor laser diodes are fixed to the heat sink, and the first pair of electrodes are attached to the second pair of electrodes. The laser diode array is capable of reducing the resistance of a wiring and improving electric properties such as threshold current and luminescent efficiency since the first pair of electrodes and the second pair of electrodes can be connected to each other without use of the wire. In particular, it is disclosed that in the laser diode array which has an array shape and includes semiconductor laser diodes arranged in parallel on the heat sink, it is possible to improve a frequency characteristic since non-uniformity of wire resistance between the laser diodes can be reduced.
FIG. 4 is a graph showing a relationship between each electrode of the exemplary laser array chip 1 shown in FIG. 3 and temperature increasing amount in operation. A fourth laser diode and a fifth laser diode are not shown since the fourth and fifth laser diodes are substantially same as a second laser diode and a first laser diode, respectively. In a case in which plural laser diodes are present within the laser array chip 1, as shown in FIG. 4, there occurs a phenomenon that operation temperatures are different between the laser diodes of the laser array chip 1. In particular, the operation temperature of a third laser diode located at the center of the laser array chip 1 is higher than that of the first laser diode and the second laser diode located in the ends thereof. A difference in the operation temperatures of the laser diodes causes an error of laser beam emitting wavelengths among the laser diodes.
With application to a laser beam printing apparatus or the like, a difference in focal distances between scanning beams occurs and a difference in locations or spot diameters of the scanning beams occurs. Therefore, printing quality of the laser beam printing apparatus may be deteriorated.